Design optimization of advanced PWR SiC/SiC fuel cladding for enhanced tolerance of loss of coolant conditions

Abstract

Limited data has been published (especially on experimental work) on integrated multilayer SiC/SiC prototypical fuel cladding. In this work the mechanical performance of three unique architectures of three-layer silicon carbide (SiC) composite cladding is experimentally investigated under conditions associated with the loss of coolant accident (LOCA), and analytically under various conditions. Specifically, this work investigates SiC cladding mechanical performance after exposure to 1,400°C steam for 48 hours and after thermal shock induced by quenching from 1,200°C into either 100°C or 90°C water. Mechanical performance characteristics are thereafter correlated with sample architecture through void characterization and ceramography. The series with a reduced thickness did not have a pseudo-ductile regime due to overloading of the composite layer. The presence of the axial tow did not yield significant difference in the mechanical behavior most likely because samples were tested in the hoop direction. While as-received and quenched samples behaved similarly (pseudo ductile failure except for one series), non-frangible brittle failure (single-crack failure with no release of debris) was systematically observed after oxidation due to silica buildup in the inner voids of the ceramic matrix composite (CMC) layer. Overall, thermal shock had limited influence on sample mechanical characteristics and oxidation resulted in the formation of silica on the inner wall of the CMC voids leading to the weakening of the monolith matrix and brittle fracture. Stress field in the cladding design is simulated by finite element analysis under service and shutdown conditions at both the core's middle height and at the end of the fuel rod. Stresses in the fuel region are driven by the thermal gradient that creates stresses predominantly from irradiation induced swelling. At the endplug, constraints are mainly mechanical. Stress calculations show high sensitivity to the data scatter and especially swelling and thermal conductivity. No cladding with the design studied here can survive either service or shutdown conditions because of the high irradiation-induced tensile stresses that develop in the hot inner monolith layer. It is shown that this peak tensile stress can be alleviated by adjusting the swelling level of the different layers. The addition of an under-swelling material such as PyC or Si can reduce the monolith tensile stress by 10%. With a composite that swells 10% less than the monolith, the stress is reduced by 20%.